Output stage especially for integrated amplifiers with externally connected output power devices

Abstract: The operation of externally connected output power transistors of a class AB amplifier is controlled without employing any external sensing resistance of the output current by driving an externally connected power transistor through a level shifting buffer and employing a limiting network composed of an integrated transistor driven by the output of a signal amplifying stage and a resistance connected in series with its drain. The buffer stage shifts the level of the driving signal of the external power transistor by a value equal to the threshold voltage of the integrated transistor of the limiting network thus ensuring the turn-off of the external power transistor under quiescent conditions.

Description: The present invention relates to a unit gain output stage employing a pair of field effect transistors (FET), as output power devices, driven in class AB mode, particularly suited for monolithically integrated power amplifiers,. A technical problem of class AB power amplifiers employing FETs, for example a pair of n-channel MOS transistors, relates essentially to the ability of effectively controlling the quiescent current flowing through the pair of output power transistors and of reducing the so-called crossover distortion. The necessity of controlling the quiescent current derives from the inevitable compromise between the opportunity of reducing power dissipation, which requires a low quiescent current I q , and of minimizing crossover distortion caused by the non-linearity of the operating characteristics of the output devices, which non-linearity may become untolerably pronounced at relatively low bias current levels. Another well known cause of crossover distortion resides in the fact that alternatively one or the other of the two output power transistors turn-off completely during a complete cycle and distortion is caused by the consequent turn-on delays that occur. Obviously the problem of crossover distortion becomes more marked at high frequency. In the relevant literature, there are many examples of circuits for controlling the bias (quiescent) current. The article entitled: "A simple direct-coupled power MOSFET audio amplifier topology featuring bias stabilization", by B. Roehr, IEEE Transactions on Consumer Electronics, Vol. CE-28, No. 4, November 1982, pages 546-552, hereinbelow cited as reference [1], and the article "Une autre conception de l'amplificateur de puissance", published in the Siliconix catalogue, AN 8 0 - 5 , pages 75-79, herein referred to as reference [2], are particularly significative. The documents NL-A-8 800 987, US-A-4 587 491, EP-A-0 309 603 and the article "A 100 V/100W, Monolithic Power Audio Amplifier in Mixed Bipolar-MOS Technology" by G. Brasca and E. Botti IEEE Transactions on Consumer Electronic Vol. 38, No. 3, August 1992, New York, US, pages 217-222, disclose various architecture of power amplifiers with control of quiescent current absorption. The article [1], describes a power amplifier wherein the biasing current of the output power devices is controlled by means of a complex system that detects the quiescent current I q , and which is based on a limiting circuit, followed by a filter. The stabilization circuit employs among other devices, an operational amplifier and a large capacitor. Significantly, the capacitor used has a capacitance that is two orders of magnitude higher than the capacitance of the other capacitors employed in the circuit, as shown at page 550. The amplifier circuit described in the document [2], is similar to the preceding one, but without the circuit for detecting quiescent current I q . In this circuit, the control of the quiescent current I q is implemented by a potentiometer, as shown in the circuit diagram at page 78. The above known solutions for implementing a control of the output stage, and particularly the fact that they require the use of either a large capacitor or a potentiometer, makes them unsuitable to be realized in a fully integrated form. Moreover, as depicted at page 79 of reference [2], the driving circuit of the two output power transistors employs PNP bipolar transistors having a relatively high current carrying capacity. Should a monolithic integration of the driving circuit be considered, the presence of PNP transistors will require a relatively large area of integration. It is generally convenient to limit the number of PNP transistors to a minimum. A step forward in improving these techiques is represented by the output stage disclosed in the European patent application No. 91121650.5, filed on December 17, 1991, by the same applicant. In that application a unit gain output stage employing a pair of power FETs, particularly suited for integrated power amplifiers, and which effectively reduces crossover distortion, irrespectively of its origin, is disclosed. Moreover, the circuit does not require the use of practically non-integratable components, such as large capacitors, potentiometers and the like, notwithstanding the fact that the circuit is perfectly capable of effectively controlling the quiescent current through the field effect output power transistors. The pertinent description contained in the above-noted European patent application is herein incorporated by express reference. In many applications, a limitation of power dissipation within the chip containing the functional circuits of the drive amplifier is required. Often, this is obtained by realizing at least one or both the output power transistors, driven in class AB, in discrete form or in any case outside the integrated circuit containing the drive amplifier. A typical example or situation of this type is that of an integrated circuit containing the drive amplifier of a so-called external Voice Coil Motor (VCM), that can be driven in class AB mode. A general objective of device manufacturers, is that of realizing the entire drive circuit in a single chip, mounted in a single package, with characteristics of substantially "universal" utility. In other words, the same IC should advantageously be usable by a manufacturer of apparatuses in a large number of applications that may contemplate different levels of the exitation current delivered to a VCM motor or the like, and therefore different values of power dissipation by simply choosing the value of external components depending upon the specific requisites of the application. The advantages that would be derived are manyfold, both in terms of a reduced cost of the package (which may be a standard low cost type, without heat sink if the output power transistors are external) and of the possibility of producing the integrated circuit for relatively large sale volumes. On the other hand, when the output power transistors are outside the device containing the integrated drive circuit and the handled power is no longer dissipated with the integrated circuit, the circuit must be capable of driving the two external power devices, while controlling their operating condition under any situation of operation, independently from their intrinsic characteristics. In fact, being the power devices external to the integrated circuit, it is necessary, according to a well known technique, to use sensors, commonly sensing resistances, of the current actually delivered to the load. The need to employ external sensing resistances of the current, is in contrast with a general requirement of reducing to the minimum the number of external components. Also in the power amplifier circuit described in the cited prior European patent application No. 91121650.5, advantageously employing a pair of field effect transistors (FETs) as output power elements should be required to realize the two output power transistors outside an integrated driving circuit, external sensing resistances of the current will be necessary in order to implement an effective control of the operating conditions of the two power FETs. SUMMARY OF THE INVENTION It has now been found and constitutes the subject matter of the present invention, an effective method and device for controlling the operation of a pair of power transistors, at least one of which is externally connected to the integrated circuit containing the driving and controlling circuitry of the two output power transistors, which advantageously does not require any external sensor of the operating condition of an externally connected power transistor or of said pair of external transistors. Basically, the invention consists in using a buffer stage for driving one of a pair of output power transistors that is connected externally to the integrated circuit and which, in case of a pair of n-channel output power transistors, will be the pull-up or upper transistor, while in the case of a pair of p-channel output transistors, will be the push-down or lower transistor of the pair. The buffer stage may be functionally connected between the output node of an amplifying stage of the driving signal and a control terminal (gate) of the driven external transistor of the output pair. The invention device further employs a limiting network comprising at least a field effect transistor, which may be directly driven by the output node of the signal amplifying stage and a resistance functionally connected in series with the transistor. The so-constituted limiting line is connected between a supply node (VCC) and the output node of the output power stage. The invention is defined in the annexed claim 1 and particularly preferred embodiments are defined in dependent claims 2 to 8. In practice, the limiting network virtually reproduces within the integrated control circuit either a pull-up or upper side transistor, in case of n-channel output transistor pair or a push-down or lower side transistor in case of p-channel output transistor pair, provided with a resistance in series with its drain terminal, while the buffer that drives the "homologous" external power transistor has the function of shifting, respectively toward ground potential or toward the supply voltage, the level of the input signal by a value equal to the threshold voltage of the field effect transistor of the limiting network, under quiescent bias conditions. In this way, under quiescent conditions, the voltage between the gate and the source of the external output power transistor is null thus ensuring the turning-off of the external power transistor, independently of the value of its intrinsic threshold voltage. When the current absorbed by the load increases, the voltage V (OP1) , present on the output node of an operational amplifier OP1 that amplifies the driving signal increases, thus bringing toward saturation the field effect transistor of the limiting network. On the other hand, since the maximum current that flows through the transistor of the limiting network is limited by the resistance in series with its drain, when the current drawn by the load becomes larger than such a maximum current that may be delivered by the transistor of the limiting network, the voltage V (OP1) on the output node of the signal amplifier OP1, will increase further and so the output voltage of a buffer BF1 that drives the external power transistor will also increase, until it reaches a voltage such as to satisfy the following relationship: V (OP1) > Vth external + V (output) thus turning-on the external power transistor. In this way, the burden of delivering the current difference between the current required by the load and the maximum current that can be delivered by the integrated transistor of the limiting network is transferred to the external power transistor. Differently from the known solutions, the circuit of the invention does not require the use of any external components for detecting the load current and advantageously may employ field effect transistors, for example DMOS transistors, capable of greatly reducing power dissipation as compared with bipolar power transistors of similar current-handling capability. By virtue of the fact that the operation of the circuit is substantially independent of the threshold voltage of the other power transistor of the output pair (n-channel or p-channel), the fact that also such other transistor of the output pair be realized within the integrated driving circuit or not becomes substantially insignificant. Therefore, depending on the contemplated application, one only or both the output power transistors may be external to the integrated circuit containing the driving and control circuitry. The different aspects and advantages of the invention will become even more evident through the following description of an important embodiment and by referring to the attached drawings, wherein: Figure 1 is a circuit diagram of an integrable unit gain output stage employing a pair of n-channel power transistors, according to the cited prior European patent application No. 91121650.5; Figure 2 is a circuit diagram of an output stage functionally equivalent to the one depicted in Fig. 1, wherein the two n-channel power transistors are connected externally to an integrated circuit which contains a modified driving circuit according to the present invention; Figure 3 is a circuit diagram of an output stage according to the present invention, similar to the stage of Fig. 2, but realized with p-channel transistors; Figure 4 is a circuit diagram of a bridge amplifier employing two output stages made according to the present invention. With reference to Fig. 1, class AB operation requires that a simultaneous ON condition of both the output power transistors M2 and M3 be ensured at a controlled current, during zero-crossing, intending for zero-crossing the situation wherein the current flowing through a load that can be connected between the output node OUT and ground is null. As already described in the prior European patent application No. 91121650.5, which is herein incorporated by express reference, in the case of a pair of n-channel power transistors, this condition is ensured by the diode-configured transistor M1, the current generator I1 and the operational amplifier OP2, which together perform a control function of the lower-side (push-down) power transistor M3. The operational amplifier OP1 represents a signal amplifying stage which is responsible of the system's gain. Its output drives the gate of the upper-side (pull-up) power transistor M2, which is connected in a source-follower configuration. The required overdrive, necessary for ensuring a rail-to-rail output voltage swing may be obtained by powering the output stage of the signal amplifier OP1 with a suitably boosted voltage that may be produced by a DC-DC converter integrated in the chip, according to a well known technique. The diode M1 permits to obtain at the drain terminal thereof a voltage, which compared with the gate voltage of the upper (pull-up) power transistor M2 by the operational amplifier OP2, controls the gate of the lower power transistor M3. When the system is in a steady state, in absence of an input signal, the circuit reaches a situation wherein the voltage difference between the inverting and the non-inverting inputs of OP2 is null. This means that also the gate-source voltages of M1 and M2 are equal. By supposing that the area ratio between M2 and M1 is N and that I1 is the current injected in the diode M1 by the current generator, the current that flows through the upper power transistor M2 is given by: I M2 =I M1 =I1·N Therefore, the quiescent current that flows through the output power stage (M2-M3) is effectively controlled. In practice, the circuit that includes M1 and OP2 behaves as a transresistance amplifier, driven by the difference between the current I and I/Nth of the current that flows through the upper power transistor M2. When the output current (conventionally intended positive when out-flowing and negative when flowing-in) becomes larger than the quiescent current, the system turns-off the lower power transistor M3. Viceversa, when the output current is negative (flowing-in), the systems reacts by turning-on the lower power transistor M3, enough to carry the output current plus the quiescent current that continues to flow through the upper power transistor M2. With reference to Fig. 2, which depicts a control circuit modified in accordance with the invention, if at least one of the output power transistors (either both output transistors M4 and M3 as shown or one only of the two power transistors, for example the pull-up transistor M4 in the case shown of an n-channel pair) is connected externally of the integrated circuit, the driving circuit is modified by introducing a buffer stage BF1 for driving the external pull-up power transistor M4 and a limiting network composed of at least a field effect transistor M2 and a resistance connected in series with its drain terminal. This limiting line is connected between the supply rail VCC and the output node OUT of the power stage. The transistor M2 may be directly driven by the output of the signal amplifying stage OP1, as shown. The buffer stage BF1 shifts toward ground potential its input voltage by a value equal to the threshold voltage of M2 under quiescent conditions. Therefore, under quiescent conditions, the gate-source voltage of the external power transistor M4, driven by the buffer, is null thus ensuring its turning-off (VGS M4 =VGS M2 -V (BF1) ) . Upon an increase of the current drawn by the load (Load), the output voltage V (OP1) of the signal amplifier OP1 increases, pushing toward saturation the transistor M2. Because the maximum current flowing through M2 is limited by the resistance R, when the current required by the load becomes larger than such a limit value I M2 , the output voltage of the signal amplifier OP1, will increase further until it satisfies the following condition: V (BF1) > Vth M4 + V (0) thus turning-on the external power transistor M4. Of course, the circuit arrangement of the invention is applicable also in the case of a circuit that employs a pair of p-channel field effect output transistors, by inverting the sign of the voltages and the sense of the currents. In this case, as will be evident to a skilled person, it will be the push-down or lower transistor M4 of the output pair, to have its gate driven by a level shifting buffer BF1 and therefore connected, through the same buffer BF1, to the gate of the integrated transistor M2 of the limiting network. Such an alternative embodiment is depicted in Fig. 3. In audio amplifiers, but not only in these types of amplifiers, it is often advantageous to drive a load through a pair of power amplifiers in a so-called bridge configuration. Also in this case, when for considerations of opportunity, one or both power transistors of the output stage of each of the two, bridge-configured, amplifiers must be external to the integrated circuit containing the respective amplifying and driving circuitry, the circuit arrangement of the invention will permit to control the operation of the externally connected power transistors without requiring external sensing elements. A basic diagram of a bridge-configured amplifier (that is of a pair of power amplifiers) wherein the output stages are realized with external power transistors (M4, M3), controlled by integrated driving circuits, modified according to the present invention, is shown in Fig. 4.

Claims: A driving circuit for a class AB output stage composed of a first and a second power transistors (M3, M4), at least the first one (M3) of which is externally connected to an integrated circuit containing said driving circuit, comprising a differential amplifier (OP2) for comparing the output voltage (VOP1) of a signal amplifying stage (OP1) with a voltage present on the anode of a diode (M1) having a cathode connected to the output node (OUT) of said output stage and directly biased by a current generator (I1) capable of forcing a quiescent current through said diode (M1) and said second transistor (M3) of said output stage that is controlled by the output of said differential amplifier (OP2), characterized by comprising further a buffer stage (BF1) for driving said first power transistor (M4), functionally connected between said output node of said signal amplifying stage (OP1) and said control terminal of said first power transistor (M4); a limiting network comprising at least a third transistor (M2) of the field effect type driven by the output of said signal amplifying stage (OP1), the source of said third transistor being connected to said output node (OUT) of the output stage; and a resistance (R) connected by a first terminal in series with the drain of said field effect transistor (M2), the second terminal of said resistance being connected to a supply node (VCC). A circuit as defined in claim 1, wherein said pair of power transistors (M3, M4) is composed of n-channel DMOS transistors. A circuit as defined in claim 1, wherein said pair of power transistors (M3, M4) is composed of p-channel DMOS transistors. A circuit as defined in claim 2, wherein said external power transistor (M4) driven by said buffer stage (BF1) is the pull-up or upper transistor of said pair (M3, M4). A circuit according to claim 4, wherein also the push-down or lower transistor (M3) of said pair is external to said integrated driving circuit. A circuit as defined in claim 3, wherein said external power transistor driven by said buffer stage (BF1) is the push-down or lower transistor (M3) of said pair (M3, M4). A circuit according to claim 6, wherein also the pull-up or upper transistor (M4) of said pair is external to said integrated driving circuit. Bridge amplifier comprising two output stages each of which is composed of a pair of power transistors (M3, M4) , at least one of which is connected externally to a respective integrated driving circuit, characterized by the fact that both said integrated driving circuits are as defined in claim 1.

Inventors: CHRAPPAN FRANCESCO, NESSI MAURIZIO, SALINA ALBERTO, CHRAPPAN, FRANCESCO, NESSI, MAURIZIO, SALINA, ALBERTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H03F1
IPC codes: H02P25, H03F3, H03F1